The present invention relates generally to a controller for a condensing unit for a refrigeration system or for other cooling systems. More particularly, the present invention relates to a condensing unit employing a variable capacity compressor which is controlled by pulse width modulation using a variable duty cycle signal derived from one or more system sensors. The condensing unit controller is capable of controlling a single evaporator or multiple evaporators of similar or mixed temperatures.
The present invention is being described associated with a refrigeration system. It is to be understood that the condensing unit of the present invention could be utilized for any other cooling system if desired.
Conventionally, refrigeration systems for refrigeration cases have employed air-cooled or water-cooled condensers fed by a rack of compressors. The compressors are coupled in parallel so that they may be switched on and off in stages to adjust the system cooling capacity to the demands of the load. Typically, the compressors and condensers are located outside of the building on the roof or in a machine room adjacent the area where the refrigeration cases are located.
Within each refrigeration case is an evaporator fed by refrigerant lines from the condensers through which the expanded refrigerant circulates to cool the case. Typically, a closed-loop control system regulates refrigerant flow through the evaporators to maintain the desired case temperatures. Proportional-Integral-Derivative (PID) closed loop control systems are popular for this purpose, with temperature and/or pressure sensors providing the sensed condition inputs.
It is common practice with retail outlets to use separate systems to supply different individual cooling temperature ranges; low temperature (for frozen foods, ice cream, nominally xe2x88x9225/F); medium temperature (for meat, dairy products, nominally +20/F); and high temperature (for floral, produce, nominally +35/to +40/F). The separate low, medium and high temperature systems are each optimized to their respective temperature ranges. Normally, each will employ its own rack of compressors and its own set of refrigerant conduits to and from the compressors, condensers and evaporators.
The conventional arrangement, described above, is very costly to construct and maintain. Much of the cost is associated with the long refrigerant conduit runs. Not only are long conduit runs expensive in terms of hardware and installation costs, but the quantity of refrigerant required to fill the conduits is also a significant cost factor. The longer the conduit run, the more refrigerant required. Adding to these added costs are environmental factors. Eventually fittings leak, allowing the refrigerant to escape to the atmosphere. Invariably, long conduit runs involve more conduit joints that may potentially leak. When a leak does occur, the longer the conduit run, the more refrigerant lost.
One solution to the above described problems is disclosed in Assignee""s U.S. Pat. No. 6,047,557, the disclosure of which is incorporated herein by reference. The solution presented in the above patent is a distributed refrigeration system in which the condenser is disposed on the refrigeration case and serviced by a special pulse-width modulated compressor that may be also disposed within the case. If desired, the condenser and compressor can be coupled to service a group of adjacent refrigerant cases, each case having its own evaporator. Further, multiple compressors with at least one pulse-width modulated compressor can be used to handle large evaporator load line-up. Also, the condenser can be disposed in a housing with the evaporator to provide a self-contained package, or can be disposed remotely, as in a split system. The pulse-width modulated compressor is driven by a control system that supplies a variable duty cycle control signal based on measured system load.
While the above described pulse-width modulated compressor and refrigeration system have performed satisfactorily, the continued development of these systems has been directed toward controlling the capacity of the compressor, the condenser and other components within the condensing unit.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.